The mining and processing of uranium bearing ores generally always utilizes a leaching process (and thus the use of a lixiviant, such as an aqueous carbonate-bicarbonate solution or an acid solution) to leach the uranium from its accompanying gangue material in the ore body. Such leaching operations may be conducted in conjunction with surface milling operations where the uranium is mined (often in open-pits) and then crushed and blended prior to leaching, or by using in-situ leaching techniques in which the lixiviant is introduced into a subterranean ore deposit and then recovered through suitable extraction systems.
The pregnant lixiviant produced during the leaching process is then processed further to concentrate the uranium therein. This further processing includes a variety of possible chemical treatments, which are usually determined by the characteristics of the specific ore being processed and also its method of extraction. For example, such further processing might include an anionic ion exchange process or solvent extraction. Regardless of the process adopted, a relatively concentrated uranium solution is produced, generally called the “eluate”, which must then be treated to precipitate a uranium compound, often referred to simply as “uranium” or a “uranium precipitate”.
Various precipitation techniques are available, one of which uses hydrogen peroxide, that are able to precipitate a uranium rich slurry for subsequent washing, dewatering and drying/calcining to produce a dry and stable uranium concentrate that can be relatively easily transported. In this respect, the downstream conversion facilities for such uranium concentrates are invariably not located near the sites of the uranium ore bodies, and thus the safe transportation, often over long distances, of the uranium concentrates renders this dry and stable form an ideal intermediate point in the overall uranium production process.
For the sake of clarity in this specification, some terms need to be defined. This is particularly the case as skilled addressees will often use the same term to represent a product at different stages in the process.
For example, the uranium concentrate referred to above as a “uranium rich slurry”, produced directly from a precipitation process, is sometimes referred to in the art as “yellowcake”, as is the dry and stable form of the uranium concentrate that exists after the subsequent washing, dewatering and drying/calcining processes. However, throughout this specification the term “uranium precipitate” will be used to describe the uranium rich slurry that is produced from the precipitation process, and where the uranium precipitate has been produced through the use of a hydrogen peroxide based precipitation process, the subsequent precipitate obtained will be referred to as a “peroxide precipitate”.
The term “yellowcake” will be used to describe the dry and stable form of the uranium concentrate produced after the uranium precipitate has been subjected to whatever subsequent washing, dewatering and drying/calcining processes are required. In this respect, it should also be appreciated that the composition of yellowcake is variable and depends upon the ore body, the lixiviant, the subsequent precipitation conditions, and the subsequent washing, dewatering and drying/calcining processes. It can consist of a mixture of, amongst other things, several ammonium-uranium-oxygen compounds, and can take different forms based upon its majority composition.
Therefore, it should be appreciated that a reference to “yellowcake” simpliciter throughout this patent specification is not to be limited only to one form of yellowcake. Indeed, yellowcake that is uranium peroxide based (UO4.nH2O, where n can vary from 2 to 4) is increasing in popularity, and this yellowcake is often referred to as a “peroxide” yellowcake. It is also possible to produce a yellowcake that is uranium trioxide based, and which is often referred to as a “trioxide” yellowcake. For the sake of simplicity and clarity, throughout this patent specification when reference is made to just “yellowcake”, the term is being used to cover all forms and types of yellowcake, with more specific references then being made to, for example, “trioxide yellowcake” or “peroxide yellowcake” where necessary.
The price paid for yellowcake by yellowcake conversion facilities is generally dependent upon the yellowcake's purity levels (purity in terms of its concentration of, for example, whichever form of uranium is required, such as the concentration of the uranium peroxide). Therefore, producers of a peroxide yellowcake generally aim to maximize the concentration of the uranium peroxide in the yellowcake, which of course requires (amongst other things) the level of impurities in the yellowcake to be minimized, and the moisture content of the yellowcake to be minimized. In terms of moisture content, ideally it would be reduced to less than about 1 wt % for any yellowcake.
Typically, the processes that have been adopted to dry a peroxide precipitate to form a peroxide yellowcake have required large quantities of water which, given the often remote locations of the uranium ore deposits, introduces significant cost issues. These processes generally involve the washing of the precipitate, which is in the form of a slurry, to remove undesirable water soluble impurities, followed by the concentration of the slurry and the subsequent reduction of its moisture content before it is packaged for sale as yellowcake.
The washing has traditionally been accomplished in a number of ways. For example, the washing function has been accomplished in a filter press or a wash thickener, avoiding the need for the addition of flocculent and improving the liquour/product contact for removal of impurities, resulting in a solids cake with a high solids concentration (greater than about 50%). The solids cake has then required moisture removal to produce the powdered peroxide yellowcake product suitable for sale for further processing. There have generally been two alternatives used, namely a rotary vacuum dryer and a continuous screw dryer, both operating at reasonably low temperatures to heat the product to about 150° C. or below, to produce a dried peroxide yellowcake.
However, with yellowcake shipment costs in many countries being based on volume rather than weight, the relatively low bulk density of the peroxide yellowcake (at about 1.4 kg/L) introduces higher transport costs than would be the case if the same peroxide precipitate could be used to produce the higher bulk density trioxide yellowcake (at about 1.8 kg/L). In the latter case, this results in transport cost savings in countries where, for example, only single level stacking of the filled drums in shipping containers is allowed. Therefore, within certain limits, even a higher production cost for the trioxide yellowcake would still be acceptable due simply to the lower transport costs of the trioxide yellowcake.
One attempt at producing the trioxide yellowcake from a peroxide precipitate has been described by Mobil Oil Corporation in U.S. Pat. No. 4,302,428 (to James M Paul). In this document, Paul states that the drying of a peroxide precipitate at low temperatures in the range of 100° C. to 300° C. results in dehydration to form a peroxide yellowcake, and that the calcining of a peroxide precipitate at temperatures in the range of 700° C. to 900° C. results in thermal decomposition to form a U3O8 yellowcake. However, Paul states that, while the trioxide yellowcake is theoretically able to be produced at temperatures in the range of 200° C. to 500° C., at some temperatures within that range (specifically between the temperatures of 300° C. and 500° C.), the conversion is accompanied by the undesirable evolution of chlorine gases.
In order to avoid the undesirable evolution of chlorine gas in such a calcining step, and still with the aim of producing the trioxide yellowcake (and not the peroxide yellowcake), Paul directs the reader to avoid temperatures above 100° C., and to instead react the peroxide precipitate with a reducing agent (such as sulfur dioxide gas) at a temperature at about room temperature, but certainly less than 100° C., in order to directly chemically convert the uranium peroxide to uranium trioxide by virtue of the reaction:UO4+xH2O+SO32−→UO3+SO42−xH2O
Paul then suggests the subsequent washing of the reduction reaction product with water (to permit removal of the water soluble salts such as sodium chloride), followed by drying of the washed slurry at a moderate temperature of less than 200° C. to form a relatively high purity trioxide yellowcake.
In a contemporaneous Mobil Oil Corporation patent, namely U.S. Pat. No. 4,293,528 (also to James M Paul) that describes efforts to thermally decompose a peroxide precipitate to form an oxide yellowcake (not a peroxide yellowcake), Paul further describes what additional reagents would be required when calcining at higher temperatures such as 300° C. (presumably to form a trioxide yellowcake), and 750° C. and 760° C. (presumably to form a U3O8 yellowcake). Indeed, Paul makes it clear that a reducing agent is required in order to react with the free oxygen evolved during calcination, again in order to retard the evolution of chlorine gas. The reducing agent is said to be ammonia or an ammonia producing compound such as ammonium carbonate.
The present invention aims to provide a process for the production of a trioxide yellowcake from a peroxide precipitate, which process does not require the addition of a reduction step nor the use of reducing agents (such as the ammonia or sulfur dioxide gases mentioned above) required for that step.
Before turning to a summary of the present invention, it must be appreciated that the above description of the prior art has been provided merely as background to explain the context of the invention. It is not to be taken as an admission that any of the material referred to was published or known, or was a part of the common general knowledge in Australia or elsewhere.